The present invention relates to microwave radiometers, and more particularly to a system for determining the microwave radiation distribution of a scene using fanbeam inversion.
There are many applications requiring a mobil microwave imaging or mapping system capable of high resolution from long distances and over a wide field of view. Among these are aerospace, meteorological, oceanographic, and astronomical applications. More specifically, microwave radiometers are well suited for: mapping terrestial, planetary, and oceanic features; measuring atmospheric water vapor, rain, and sea surface temperature; and assessing hydrographic phenomena and surface conditions below clouds or rain.
By way of example, present meteorological radiometric mappers are limited to visible and infrared frequencies which at best weakly penetrate cloud cover. Thus, present systems are prevented from obtaining continuous observation of the Earth's surface. Portions of the microwave radiation spectrum (1 mm to 1 m wavelengths, herein) readily penetrate cloud cover. Thus, a microwave radiometer could provide all-weather continuous satellite imaging of the Earth's surface for meteorological and other applications.
Microwave imaging presents a challenge in terms of instrument size and weight. For most of the applications listed above, antenna dimensions on the order of ten meters and more are required. Clearly, standard "dish" antennas are cumbersome to build and deploy. For example, a microwave imaging system in geosynchronous orbit capable of matching the available fifty meter resolution of infrared and visible imagers might require a dish antenna one kilometer in diameter and might weight 4,000,000 pounds.
One method, classical aperture synthesis or CAS, provides for limiting size and weight on Earth-based systems. In the CAS systems first built, two or more antenna elements are moved relative to each other to span a constructive aperture of the desired dimensions. An image can be reconstructed from the spatial frequency spectra thus obtained from the various relative positionings. More recently, CAS systems have employed arrays of elements which are cross-correlated in various combinations.
A disadvantage of CAS is the requirement of an array support system or track system to control the movement of the antenna elements. This is most serious in situations where the antenna must operate without the physical support of the Earth or other support structure vary large relative to the antenna; the array support or track may weigh as much as a filled aperture array with comparable performance. Furthermore, the sensitivity of a CAS system is far below that of a comparable full-aperture system. The low sensitivity increases the time required for forming an image. Another disadvantage of CAS is the power required for the relative movements. These disadvantages are particularly significant in geosynchronous meteorological applications.
With the advent of computers, the computation power and new mathematical techniques for reconstructing images from data collected by line-source antennas became available. These techniques were applied by radio astronomers who built large linear antenna arrays. The data gathered by these antennas as the Earth rotates is collected and then processed to form an image of a star field.
Accordingly, viable imaging systems have been built which are well adapted to determining the source direction of radio waves where the source is effectively a point source an astronomical distance away. On the other hand, such systems have several limitations. They do not create precise images when the target covers a wide field of view, such as an earth meteorological scene when viewed from a satellite. Secondly, the images they provide are distorted as a function of angular displacement of the target from the Earth's polar axis. Thirdly, such an antenna cannot view stars in the opposite polar hemisphere. Additionally, such antennas lose resolving power as the imaged area approaches the equatorial plane.
Another disadvantage is that the time required for a complete image reconstruction is limited by the revolution rate of the Earth to about twelve hours. Furthermore, the Earth cannot be a target of these Earth-bound antennas for two reasons: one cannot reconstruct an image of a scene that rotates at the same rate as the line-source antenna, and the Earthbound antennas cannot be positioned readily to view the Earth from a useful distance. Finally, these antennas cannot be transported readily to look at disparate targets and cannot be incorporated practically in aerospace and terrestial vehicles.
Furthermore, since they are located far from the Earth's poles, these antennas rotate about an axis thousands of miles from the antennas. Therefore, the distance between the axis of rotation and the antenna is much greater than any physical dimension of the antenna itself. This proves unwieldy in a portable design and introduces parallax errors unacceptable for most non-astronomical applications. Since the size and weight of these line-source antennas are proportional to the diameter of the equivalent two-dimensional antenna, rather than the square of that diameter, considerable materials and related savings accrued in the construction of these antennas, one of which was about 1700 ft long.
What is needed for meteorological and other applications is a transportable and/or orbitable microwave radiometer which can image independent of the Earth's rotation rate and over a relatively near range and a wide field of view.